Apparatus for the Aftertreatment of the Exhaust Gases of Diesel Engines

ABSTRACT

The subject matter of the invention is apparatuses for the aftertreatment of the exhaust gases of diesel engines. The apparatus comprises a pipe ( 2 ), through which the exhaust gases flow, a nozzle for injecting an additive into the pipe ( 2 ), and a mixing section ( 3 ) which is arranged behind the nozzle. A mixing pipe ( 20 ) is inserted into the pipe ( 2 ) adjacently to the nozzle. Said mixing pipe ( 20 ) is provided with large-area perforations ( 22 ) on the circumference, is seated in the pipe ( 2 ) with a radial spacing, has a closure cover ( 25 ) at its end with a receptacle ( 26 ) for the nozzle and, at its other end, has a funnel-shaped widened portion ( 23 ) which bears against the pipe ( 2 ) at least in regions.

TECHNICAL BACKGROUND

The invention relates to devices for aftertreatment of exhaust gases ofdiesel engines according to the preamble of Claim 1.

PRIOR ART

Modern aftertreatment methods for the exhaust gases of diesel enginesrequire the addition of additives to the exhaust gas to allow therespective chemical reactions to take place with optimal effect. Thus,to improve and maintain selective catalytic reduction in the so-calledSCR method, ammonia, usually in the form of an aqueous urea solution,must be added to the exhaust gas in a downstream SCR catalyst.Hydrocarbons are added to the exhaust gases to improve and maintain thecatalytic function of a diesel oxidation catalyst. Hydrocarbons are alsoadded to the exhaust gases to initiate regeneration of a loaded dieselsoot particulate filter.

In many of the aforementioned cases, the additives are injected into theexhaust gas stream in an extremely finely divided form. Then a so-calledmixing zone follows downstream from the nozzle, where the exhaust gasesand the additives are to be mixed as homogeneously as possible. Whenusing an aqueous urea solution as the additive in particular, thismixing zone must be long enough so that sufficient time remains for thewater to evaporate completely.

In addition, it is necessary to ensure that the additives do not come incontact with the pipe wall until it is completely evaporated. Since thepipe wall is usually colder than the exhaust gas itself, the additivewould be deposited there. This might result in attack on the pipe but inparticular then the proper amount of additive would be missing from theexhaust gas aftertreatment reaction, which would then only take placeincompletely. This is unsatisfactory.

The technical world has of course attempted to remedy thesedisadvantages. In particular it has been proposed that static mixingelements in the form of blades set at a fixed angle, plates and the likeshould be installed in the mixing zone to achieve a thorough mixing ofthe two even with a short mixing zone by creating turbulence in themixture of exhaust gas and additive. Unfortunately, these known proposedapproaches have entailed high costs and especially high backpressures.Therefore, there continues to be a substantial demand for technicalapproaches which will eliminate the disadvantages of the prior art.

EXPLANATION OF THE INVENTION

The object of the present invention is to provide a device foraftertreating the exhaust gases of diesel engines, said device beingcapable of achieving complete evaporation of additives even with a shortmixing zone and the most homogeneous possible mixing of exhaust gas andadditives as well as preventing additives from being deposited on theinside wall of the pipe of the mixing zone.

This object is achieved by a device having the features of Claim 1.

It is an essential advantage of the present invention that the exhaustgases enter simultaneously into the mixing pipe from all sidesimultaneously through the perforations distributed on the circumferenceof the jacket of the pipe so that the exhaust gases flowing through themixing pipe are concentrated at the center of the mixing pipe. Theexhaust gases concentrated at the center leave the mixing pipe togetherwith the additive that has been injected and then flow through themixing zone, where the concentration near the center is upheld so thatthe additives are not deposited on the wall of the mixing zone.

According to the one embodiment of the invention, the widened portion ofthe mixing pipe is in close contact with the pipe carrying the exhaustgas over the entire circumference, and the conically-shaped part of thewidened portion is provided with perforations over a large area. Theseperforations are in the area of the wall of the downstream mixing zonebased on their positioning, so the part of the exhaust gases flowingthrough these perforations forms a flow near the wall which additionallyprevents additives from being able to come out of the mixture flowing atthe center of the mixing zone and reach the wall of the mixing zone.

According to one embodiment of the invention, the pipe carrying theexhaust gas is a bend, preferably a 90° bend.

According to an advantageous further embodiment of the invention, theperforations are irregularly distributed in the mixing pipe. Throughskilled distribution of these perforations, it is possible to controlthe quantity of exhaust gases flowing into the mixing pipe distributedaround the circumference thereof in a targeted manner so that even withunfavorable external flow conditions, such as those which must beexpected with a tight 90° bend, for example, the desired centering ofthe mixture of exhaust gas and additive is always maintained.

BRIEF DESCRIPTION OF THE DRAWINGS

On the basis of the drawings, the present invention will now beexplained in greater detail in the form of one exemplary embodiment, inwhich:

FIG. 1 shows an isometric diagram of a mixing pipe,

FIG. 2 shows the mixing pipe from FIG. 1, installed in a 90° pipe bend,

FIG. 3 is an isometric diagram showing a distribution of the mixture ofexhaust gas and additive in a mixing zone, and

FIG. 4 shows a simplified diagram of the exhaust gas flows out of thepipe carrying the exhaust gas into the mixing pipe.

FIG. 1 shows an isometric diagram of a mixing pipe 20. The mixing pipe20 is open at one end 21, with a conical or bell-shaped widened portion23 at the other end. The pipe jacket is provided with large-areaperforations 22. Another ring of perforations 24 is provided on theconical part of the widened portion 23.

FIG. 2 shows the mixing pipe 20 inserted into a pipe 2 carrying exhaustgases, represented by an arrow 1, here in the form of a 90° pipe bend.The open end 21 of the mixing pipe 20 is provided with a closing cover25, which has a receptacle 26 for a nozzle (not shown) which injects anadditive into the mixing pipe 20 and/or the exhaust gases 1. The nozzlemay inject an aqueous urea solution, hydrocarbons or other additivesinto the exhaust gas 1 as needed.

As FIG. 2 also shows, the widened portion 23 on the end of the mixingpipe is of such dimensions that it is in close contact with the insideof the pipe 2 carrying the exhaust gas. In this way, the exhaust gas 1flowing into the pipe must flow through the perforations 22, 24. Thepart of the exhaust gases flowing into the perforations 22 in the pipejacket leads to a concentration of the exhaust gas flow at the center ofthe mixing pipe 20 and the mixing zone 3, which follows the latter (FIG.3). However, the part of the exhaust gases flowing through theperforations 24 forms a jacket flow in the downstream mixing zone 3,which effectively protects the mixture of the exhaust gas and theadditive from coming in contact with the wall.

FIG. 3 shows the diagram of the flow paths of the mixture of exhaust gasand additive through the mixing pipe 20 and the downstream mixing zone 3according to a simulation. The flow paths form a helical eddy but do notreach the wall of the mixing zone 3, so nothing is deposited on thewall.

FIG. 4 shows purely schematically a cross section through the pipe 2carrying the exhaust gas and through the mixing pipe 20 in a simplifieddiagram of the exhaust gas flow 1. It can be seen here that based on thesymmetrical flow around the mixing pipe 20, a double eddy 1′ develops inthe interior of the mixing pipe 20, ensuring a homogeneous mixing of theexhaust gas and the additive on the one hand and on the other handensuring the concentration of the mixture of exhaust gas and additive atthe center of the mixing pipe 20 and the downstream mixing zone 3.

1. A device for aftertreating the exhaust gases (1) of diesel engines,comprising a pipe (2) through which the exhaust gases (1) flow, a nozzlefor injecting an additive into the pipe (2) and/or into the exhaustgases (1), and a mixing zone (3) downstream from the nozzle, and amixing pipe (20) inserted into the pipe (2) at a radial distance fromthe pipe (2) at the connection to the nozzle, the mixing pipe (20) has aconical or bell-shaped widened portion (23) on one end, in contact withthe pipe (2) at least partially and being provided with a ring oflarge-area perforations (24), characterized by the features: the mixingpipe (20) is cylindrical has a pipe jacket provided with large-areaperforations (22) has a closing cover (25) with a receptacle (26) forthe nozzle on its end, on its other end has a conical or bell-shapedwidened portion (23) which is in contact with the pipe (2) in at leastsome areas the exhaust gases (1) flow around it symmetrically and itforms a double eddy (1′) in its interior.
 2. The device according toclaim 1, characterized by the feature: the widened portion (23) is intight contact with the pipe (2) over the entire circumference.
 3. Thedevice according to claim 1, characterized by the feature: the pipe (2)is a bend, preferably a 90° bend.
 4. The device according to claim 1,characterized by the feature: the perforations (22, 24) of the mixingpipe (20) are irregularly distributed.
 5. The device according to claim1, characterized by the feature: the nozzle is a urea nozzle.
 6. Thedevice according to claim 1, characterized by the feature: the nozzle isa hydrocarbon nozzle.